Toxoplasma gondii infects 1,500,000 human subjects in the US each year, and about 750,000 infections are thought to occur via the consumption of contaminated meat products. It is estimated that about 225,000 clinical cases of acute toxoplasmosis occur annually, and one-third of those reported require extended hospitalization. As such, toxoplasmosis represents the third-leading cause of food-related mortality in the US. Many of the Toxoplasma food-borne deaths occur in patients that are immunocompromised (unborn and infants, cancer/transplant patients, and those with AIDS) where the ability of this organism to switch between the tachyzoite and bradyzoite stages underlies the pathogenesis of disease. It is a common goal of NIAID projects RO1 AI44600 and R37 AI28724 to understand the molecular basis of bradyzoite development. Since current treatment therapies are ineffective against the tissue cyst, and may exacerbate the progression of chronic disease during treatment of acute toxoplasmosis, we believe a study of the molecular basis for bradyzoite development will yield new relevant drug targets and provide the basis for more efficacious drug therapies. In the original applications, we proposed differential display to determine changes in gene expression during tissue cyst formation to meet this objective. However, this method has many limitations, not the least of which is a high frequency of false positives, and has now been replaced by improved functional genomic methods including the novel 5'-3'-SAGE strategy (serial analysis of gene expression) described in this proposal. Unfortunately, there is no provision in the existing projects that would permit us to take advantage of new sequencing-based technologies to study gene expression. Therefore, in this R21 application, we propose a focused SAGE analysis of gene expression during bradyzoite development. It is our hypothesis that the genes and pathways identified here (particularly early responsive genes) will be key to unlocking the molecular correlates directing the tachyzoite to bradyzoite switch mechanism(s), and the future development new therapeutic regimens.